In semiconductor device fabrication, the various processing steps fall into four general categories: deposition, removal, patterning, and modification of electrical properties. Deposition is any process that grows, coats, or otherwise transfers a material onto the wafer. Available technologies consist of physical vapor deposition (PVD), chemical vapor deposition (CVD), electrochemical deposition (ECD), molecular beam epitaxy (MBE) and more recently, atomic layer deposition (ALD) among others. Removal processes are any that remove material from the wafer either in bulk or selective form and consist primarily of etch processes, both wet etching and dry etching such as reactive ion etch (RIE). Chemical-mechanical planarization (CMP) is also a removal process used between levels. Patterning covers the series of processes that shape or alter the existing shape of the deposited materials and is generally referred to as lithography. For example, in conventional lithography, the wafer is coated with a chemical called a “photoresist.” The photoresist is exposed by a “stepper,” a machine that focuses, aligns, and moves the mask, exposing select portions of the wafer to short wavelength light. The unexposed regions are washed away by a developer solution. After etching or other processing, the remaining photoresist may be removed by plasma ashing.
Semiconductor manufacturing entails the formation of various patterns on wafers. These patterns define the structure of and interconnection between the different components and features of the integrated circuit. The patterns are formed on wafers using patterning tools known as masks and reticles.
A mask is defined as a tool that contains patterns which can be transferred to an entire wafer or another mask in just a single exposure. A reticle is defined as a tool that contains a pattern image that needs to be stepped and repeated in order to expose the entire wafer or mask. Reticles have two major applications: 1) printing of images directly onto wafers in equipment known as step-and-repeat aligners; and 2) printing of images onto masks which, in turn, transfer the images onto wafers. The patterns on a reticle are usually 2× to 20× the size of the patterns on the substrate. However, some reticle patterns are 1× the substrate pattern.
The equipment used for printing patterns on substrates that are smaller than the patterns on the reticles is also referred to as a ‘reduction stepper’, while one that's used for printing equal-size patterns is known as a 1× stepper. The ‘polarity’ of a mask or reticle can either be positive or negative. A positive mask or reticle has background areas (or fields) that are clear or transparent, which is why a positive mask or reticle is also known a ‘clear-field’ tool. A negative mask or reticle has fields that are opaque, which is why a negative mask or reticle is also known a ‘dark-field’ tool.
There are many ways by which a pattern may be transferred to a wafer using a mask, a reticle, or a combination of both. Regardless of the pattern transfer process, everything starts with a set of pattern data that are converted into an actual pattern by a ‘pattern generator.’ Commonly-used pattern generators include: 1) plotters; 2) optical pattern generators; and 3) electron beam pattern generators.
The patterns generated by the pattern generators are formed on either a mask or reticle. For example, plotter-generated patterns can be photo-reduced and formed on 10× emulsion reticle, while optically generated patterns can be formed on 5-20× hard-surface reticles. E-beam generated patterns can be formed on a 5-10× reticle, a 1× reticle, a 1× hard surface mask, or even directly to the wafer.
The patterns formed on a reticle can be transferred directly onto the wafer, or they may first go to a mask which is the one that transfers the patterns to the wafer. Patterns on masks generally get transferred to the wafer directly.
Currently reticles are patterned with fixed images which will block light in certain regions while allowing light to penetrate other regions. This allows an image of the design to be translated to the silicon through a lithographic system. Reticles are the source image for generating patterns in semiconductor processing. They suffer from high cost and slow turn-around times required when first fabricated or modified. In addition, each reticle is a fixed design which means that any changes in a design require a new reticle to be fabricated.
Therefore, there is a need for a multi-use reticle that may be programmed using electrical signals. This would allow for more flexible designs, a more rapid turn-around time from concept to test and an overall reduction in system costs.